Lidar system with speckle mitigation

ABSTRACT

LIDAR systems, and methods of measuring a scene are disclosed. A laser source emits one or more optical beams. A scanning optical system scans the optical beams over a scene and captures reflections from the scene. A measurement subsystem independently measures the reflections from N subpixels within each scene pixel, where N is an integer greater than 1, and combines the measurements of the reflections from the N subpixels to determine range and/or range rate for the pixel.

RELATED APPLICATION INFORMATION

This application is a continuation-in-part of application Ser. No.16/032,535, filed Jul. 11, 2018, titled PRECISELY CONTROLLED CHIRPEDDIODE LASER AND COHERENT LIDAR SYSTEM, which claims priority fromprovisional patent application No. 62/675,567, filed May 23, 2018,titled HIGH-SPEED COHERENT LIDAR, and provisional patent applicationSer. No. 62/536,425, filed Jul. 24, 2017, titled COHERENT CHIRPED LIDARFOR 3D IMAGING, the contents of all of which are incorporated byreference herein in their entirety.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to three-dimensional (3D) image systems and,specifically, to coherent Light Detection and Ranging (LIDAR) imagingsystems.

Description of the Related Art

3D imaging systems are critical for a number of applications in theautomotive industry, robotics, unmanned vehicles etc. Traditionally,pulsed LIDAR systems relying on the time of flight (TOF) technique havebeen investigated for these applications, wherein the range to a targetis determined by measuring the time taken by a single narrow pulse to bereflected from a distant target. TOF LIDAR suffers from some majorchallenges and limitations. Since the amount of light reflected fromdistant targets is very small (typically under 100 photons in themeasurement time) and the pulse width needs to be very small (<1 ns) toachieve high range accuracy, these systems require sophisticateddetectors, typically high-speed photon-counters. The TOF techniqueplaces very stringent requirements on the dynamic range of the detectorand associated electronics (typically about 60 dB, or 20 bits). Further,these very sensitive detectors often have difficulty dealing withcrosstalk from other LIDAR systems or from other sources of light(including direct sunlight) when operated in real-world situations.

Coherent LIDAR is a promising 3D imaging technology because of itspotential to achieve excellent performance on a number of key metrics:high-speed (>1 million points/second), long-range (>200 m for targetswith albedo of 0.1), high lateral resolution (<0.1 degrees), and finerange precision (<10 cm). FIG. 1 is a simplified block diagram of anexemplary coherent LIDAR system 100. The LIDAR system 100 is based onthe frequency modulated continuous wave (FMCW) technique, where thefrequency of a continuous wave (CW) laser is “chirped” or changed inaccordance with a predetermined periodic frequency versus time function.The periodic frequency versus time function may be a positive linearsawtooth function where the frequency starts at a baseline value,increases linearly over a time period, resets to the baseline value, andrepeats periodically. The periodic frequency versus time function may bea negative linear sawtooth function where the frequency starts at abaseline value, decreases linearly over a time period, resets to thebaseline value, and repeats periodically. The periodic frequency versustime function may be a linear triangular function where the frequencystarts at a baseline value, increases linearly over a time period,decreases linearly back to the baseline value over a second time period,and repeats periodically. The periodic frequency versus time functionmay be some other linear or nonlinear function. The system 100 includesa semiconductor diode laser 100 to produce the chirped waveform. Thisdevice will be referred to in this patent as a Chirped Diode Laser(CHDL).

The output of the CHDL 110 is divided into two components by an opticaltap 120. The optical tap 120 may be a tap coupler as shown, anasymmetric beam splitter, or some other component effective to separatea small fraction of the CHDL output to be used as a Local Oscillator(LO) wave 125. The majority (typically >90%) of the CHDL output power isdirected to a target 160 via a circulator 130. Typically, an opticalsystem (not shown), such as a telescope, is used to form the CHDL outputinto a collimated output beam 135 that illuminates the target 160. Lightreflected from the target 140 is collected by the same optical system(not shown) and returned to the circulator 130. The reflected lightexits the circulator 130 and is combined with the local oscillator wave125 in an optical combiner 145. The optical combiner may be a 2×1coupler as shown, a beam splitter, or another component to combine theLO wave and the reflected light from the target. The combined LO waveand the reflected light from the target are incident on a photodetector(PD) 150. The photodetector 150 provides an output current proportionalto the incident optical power. The photodetector 150 effectivelymultiplies the amplitudes of the reflected light and the LO wave tocreate a coherent “beat signal” whose frequency is directly proportionalto the round-trip time delay to the target, and the range to the targetis thus determined. The data processing function 155 extracts the beatsignal from the photodetector output to determine the range and/or rangerate to the target.

While FIG. 1 is drawn is if the optical paths between the CHDL,couplers, circulator and photodetector are optical fibers, this is notnecessarily the case. The couplers and circulator can be implementedusing discrete optical elements and the optical paths between theseelements may be in free space.

FIG. 2A is a graph of optical frequency versus time which illustratesthe operation of a coherent LIDAR system, such as the coherent LIDARsystem 100. In this example, the optical frequency of the output wavefrom a chirped diode laser (CHDL) follows a positive linear sawtoothfunction where the optical frequency during each chirp period T is givenby

ω=ω0+ξt,  (1)

-   -   where        -   ω=optical frequency of the laser output wave;        -   ω0=a baseline frequency at the start of each chirp;        -   ξ=a chirp rate measured in frequency per unit time; and        -   t=time.

The optical frequency of the reflected wave from a target follows asimilar function, but is offset in time from the output wave by a periodgiven by

τ=2R/c  (2)

-   -   where        -   τ=the time interval between the output and reflected            chirps=the round-trip time to the target;        -   R=the range the target; and        -   c=the speed of light.            The chirp period T must be longer than the round trip time            to the target τ to provide a measurement interval T_(M). The            required length of the measurement interval T_(M) is            determined by, among other factors, the signal-to-noise            ratio of the reflected beam. At any given time during the            measurement interval T_(M), the frequency difference Δω            between the output and reflected waves is given by:

Δω=ξτ.  (3)

FIG. 2B is a graphical representation of the mixing of a LO wave and areflected wave incident on a photodetector. The output current from thephotodetector is given by

i≈(P _(LO) P _(Ref))^(1/2) cos(Δωt+ω ₀τ)  (4)

-   -   where i=current output from the photodetector; and    -   P_(LO), P_(REF)=power of the LO and reflected waves.        Δω can be determined by processing the output current from the        photodetector. For example, the current value may be digitized        with a sample rate substantially higher than the anticipated        value of Δω. A Fourier transform or other process may be        performed on the digitized samples to determine Δω.        Alternatively, Δw may be determined by a bank of hardware        filters or some other technique. The range R to the target may        then be determined by

R=Δωc/2ξ  (5)

-   -   where c=the speed of light.        The resolution of the range measurement is given by

$\begin{matrix}{{\delta \approx {{c/2}B}}{{{{{where}\mspace{14mu} \delta} = {{resolution}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {range}\mspace{14mu} {direction}}},{and}}B = {{{frequency}\mspace{14mu} {change}\mspace{14mu} {during}\mspace{14mu} {chip}\mspace{14mu} {period}\mspace{14mu} {in}\mspace{14mu} {cycles}\text{/}{\sec.}}\mspace{14mu} = {\xi \; {T/2}{\pi.}}}}} & (6)\end{matrix}$

A coherent LIDAR system can determine both a range to a target and arate of change of the range (range rate) to the target. FIG. 3 is agraph of optical frequency versus time which illustrates determining arate of change of the range to a target. In this example, the output ofthe laser source is modulated with a triangle function, where theoptical frequency increases linearly from ω0 to ω0+2πB over a timeperiod T and then decreases linearly back to ω0 over the next period T.The reflected beam from a moving target will be subject to a Dopplerfrequency shift given by

ω_(D)=2π/λ(dR/dt)  (7)

-   -   where        -   ωD=Doppler frequency shift in the reflected beam;        -   λ=the laser wavelength; and        -   dR/dt=the rate of change of the range to the target.

When a target is illuminated with a laser beam with an up-chirp (i.e., abeam that increases in frequency with time as in the first half of thetriangle function), the Doppler shift and the frequency shift due to thedelay of the reflected beam are additive, such that

Δω⁺ =ωD+ξτ;  (8)

-   -   where Δω⁺=the frequency difference between the output and        reflected waves for an up-chirp.        When a target is illuminated with a laser beam with a down-chirp        (i.e., a beam that decreases in frequency with time as in the        second half of the triangle function), the Doppler shift and the        frequency shift due to the delay of the reflected beam are        subtractive, such that

Δω⁻ =ωD−ξτ;  (9)

-   -   where Δω⁻=the frequency difference between the output and        reflected waves for a down-chirp.        Illuminating a target with both an up-chirp beam and a        down-chirp beam, concurrently or sequentially, allows        determination of both range and range-rate. For example, the        optical frequency of a single CHDL can be modulated to follow a        linear triangular function, as shown in FIG. 3, to provide        sequential up-chirp and down-chirp measurements. Simultaneous        determination of range and range rate is a major advantage of        coherent LIDAR systems that enables faster and better target        tracking in a variety of applications.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a coherent LIDAR system.

FIG. 2A is a graph of optical frequency versus time illustrating theoperation of a coherent LIDAR system.

FIG. 2B is a graphical representation of the output current from aphotodetector.

FIG. 3 is a is a graph of optical frequency versus time illustratingdetermination of range and range-rate based on Doppler shift.

FIG. 4 is a block diagram of a state machine chirped diode laser.

FIG. 5 is a block diagram of a coherent LIDAR system using two chirpedlasers to concurrently measure range and range rate.

FIG. 6 is a graph of optical frequency versus time illustrating multiplemeasurements during a single laser chirp.

FIG. 7 is a graphical representation of an amplified chirped laser forlong range LIDAR systems.

FIG. 8 is a block diagram of a scanning coherent LIDAR system.

FIG. 9 is a block diagram of a scanning optical subsystem for use in acoherent LIDAR system.

FIG. 10 is a block diagram of another scanning optical subsystem for usein a coherent LIDAR system.

FIG. 11 is a block diagram of another scanning optical subsystem for usein a coherent LIDAR system.

FIG. 12 is a block diagram of another scanning optical subsystem for usein a coherent LIDAR system.

FIG. 13A is a graphical representation of temporal over-sampling.

FIG. 13B is a graphical representation of spatial over-sampling.

FIG. 14 is a block diagram of a coherent LIDAR system using spatialoversampling.

FIG. 15 is a block diagram of a scanning optical subsystem for use in acoherent LIDAR system with spatial over-sampling.

FIG. 16A is a block diagram of a single element is a staring coherentLIDAR system.

FIG. 16B is a block diagram of a staring LIDAR system.

FIG. 17 is a depiction of a staring LIDAR sensor array integrated on asingle chip.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the most significantdigit or digits provide the number of the figure where the element isintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

The key requirement for a coherent chirped LIDAR system is a laser whoseoptical frequency varies with time in a precisely controlled fashion.LIDAR systems commonly incorporate semiconductor diode lasers andattempt to control the laser to produce a precisely linear chirped wave.However, the principles described in this patent can be more generallyapplied to any type of laser whose output frequency can be varied bychanging one or more input parameters. These principles can also beapplied to generating nonlinear chirps.

Feedback-controlled chirped diode lasers measure the frequency outputcharacteristic of the laser and use the measurement to provideclosed-loop feedback to control the laser output frequency. However,measuring and controlling the rate of change of the laser outputfrequency typically requires a finite time interval. For example, afraction of the laser output power may be transmitted through anunbalanced (or asymmetric) Mach-Zehnder interferometer (MZI) and onto aphotodetector. The output frequency of the beat signal produced by theMZI-photodetector is directly proportional to the slope of the frequencychirp (i.e., the rate of change of frequency with time). By comparingthis beat frequency to a desired beat frequency (corresponding to adesired frequency chirp rate), an error signal can be generated and fedback to the laser (typically after additional filtering). Thisclosed-loop system can generate a precisely controlled linear chirp, butonly works well if the chirp duration and the LIDAR measurement time aresubstantially longer that the time interval needed to measure andcontrol the slope of the frequency chirp.

The chirp rate of high-speed coherent LIDAR systems is dictated by therequired resolution and image update rate. Some systems may use chirpdurations or measurement times less than 1 microsecond. Closed-looplaser control, as described in the preceding paragraph, does not workfor these high speed LIDAR systems, because the propagation delays inthe feedback system (such as optical delays in the unbalanced MZI andthe response time of the laser and any loop filters) are comparable toor larger than the chirp duration itself.

The transmitter output in a coherent LIDAR system is a sequence ofidentical (at least in theory) frequency chirps that repeatperiodically, as shown in FIG. 2A. Environmental fluctuations, such astemperature changes, that cause the laser performance to change withtime typically have a time constant that is much larger than the chirprepetition period. Thus it is possible to control a future chirp basedon measurements taken on one or more previous chirps, so long as thedelay of the control system is smaller than the time constant of anyenvironmental fluctuations.

FIG. 4 is a block diagram of a frequency modulated laser 400 suitablefor use in a high speed LIDAR system. The system includes a laser device410 that is driven by a laser driver circuit 415 that controls thefrequency of the laser output. The laser device 410 may be a diodelaser, in which case the laser driver 415 controls the output frequencyof the laser 410 by varying an electrical current provided to an inputof the laser 410. The laser device 410 may be some other type of laser,in which case the laser driver 415 may control the output frequency ofthe laser 410 by varying one or more other parameters.

A portion of the output beam from the laser 410 is extracted by anoptical tap 420 and applied to an optical frequency discriminator 425that provides a measurement of the rate of change of the outputfrequency of the laser output. The optical tap 420 may be a tap coupleras shown, an asymmetric beam splitter, or some other device thatextracts a small portion of the laser output. The optical frequencydiscriminator may be, for example, an asymmetric MZI and photodetectoras shown in FIG. 4. In this case, the output of the photodetector is asignal having a frequency proportional to the rate of change of thelaser frequency (the asymmetric MZI and photodetector operate exactly asdescribed for the coherent LIDAR system 100, with the “range” to thetarget determined by the difference in the length of the two legs of theMZI). When the output of the laser 410 is a perfectly linear chirp, thesignal output from the photodetector will be a constant predeterminedfrequency. When the output of the laser deviates from a linear chirp, acorresponding deviation will occur in the frequency of the signal outputfrom the photodetector. A technique other than an asymmetric MZI andphotodetector may be used for the optical frequency discriminator 425.

An error determination module 430 receives the output from the opticalfrequency discriminator 425, and determines the deviation of the laserfrequency from its intended value as a function of time during the chirpperiod. The error determination may be performed by hardware and/or by aprocessor executing a method, such as a Hilbert transform, implementedin software. The error determination for the present chirp (“chirp k”)is provided to a correction determination module 435 that determines acorrection to be applied to the drive signal for a subsequent chirp k+1or, more generally, a future chirp period, where the delay between themeasurements and the future time period is less than the time constantof any environmental fluctuations. The correction module 435 maydetermine the correction to be applied to the future chirp usinghardware and/or a processor executing a method implemented in software.The correction determination module 435 may determine the correction tobe applied to the future chirp based upon the error determination and/orthe correction for one or more prior chirps. For example, the correctiondetermination module 435 may determine the correction to be applied tothe future chirp based, at least in part, on a weighted sum or weightedaverage of the determined errors for two or more prior periods of theperiodic frequency versus time function. The correction determinationmodule 435 may determine the correction to be applied to the futurechirp based, at least in part, on digital or analog filtering of thedetermined errors for one or more prior periods of the periodicfrequency versus time function.

The laser 400 will be subsequently refer to as a “state machine CHDL”because where the information about the current “state” of the chirp isfed back to influence the future state of the chirp.

Although not shown in FIG. 4, the laser frequency error can bedetermined from multiple and adaptively varying measurements and/orfilters. For example, the free spectral range of the asymmetric MZI maybe varied to measure large discrepancies when the system is turned on(or when a parameter such as temperature is changed), but switched tohigher sensitivities to measure small errors when the system is at ornear steady state. Further, the drive signal fed into the laser may alsoinclude open-loop pre-distortion to compensate for known nonlinearitiesin the laser characteristics and/or environmental compensation for knownchanges to the environment, such as a separate measurement of thetemperature of the LIDAR system (which generates a known shift in laseroperating parameters). The use of open-loop pre-distortion and/orenvironmental compensation may allow the laser 400 to reach a steadystate quicker when the system is turned on or when an environmentalparameter such as temperature is changed.

As illustrated in FIG. 3, the time required to obtain the range (R) andrange rate (dR/dt) information for a single target pixel (pictureelement) using a conventional FMCW approach is 2T, where the one-sidedchirp duration T is the sum of the round trip time delay τ=2R/c and themeasurement time T_(M). The signal to noise ratio (SNR) required forgood target detection determines the value of T_(M), and T_(M) can bereduced by adjusting system parameters such as CHDL power or receivercollection aperture. After a time (2T), the beam is scanned to the nextpixel on the target. The total number of pixels that can be measured ina given time, termed the “3D imaging rate” (3D-IR) is (½T) pixels persecond.

It is desirable that the 3D-IR be as high as possible. For the standardsystem of FIG. 1, even if the measurement time T_(M) is made very small,the 3D imaging rate is limited by the maximum round trip transit time inthe scene, i.e.,

3D-IR<½τ_(max) =c/4R _(max).  (8)

This is a limitation imposed by the finite speed of light c. Forexample, for a maximum range of 300 m, the 3D imaging rate is limited to0.25 million pixels per second. This limitation may not be acceptable insome applications.

The “speed of light limit” on the 3D imaging rate can be overcome withtwo improvements to the basic coherent LIDAR system of FIGS. 1, 2, and3. The first improvement, as incorporated into the LIDAR system 500 ofFIG. 5, is to use two CHDLs 510, 570 to simultaneously illuminate thesame pixel on the target, with the frequencies of the CHDLs chirping inopposite directions. For example, the frequency of the first CHDL 510may follow a positive sawtooth function and the frequency of the secondCHDL 570 may follow a negative sawtooth function. Alternatively, thefrequency of the first CHDL 510 may follow a triangle function and thefrequency of the second CHDL 570 may follow a triangle function shifted180 degrees (shifted in time by one chirp period T) compared to thefirst CHDL. This enables the up and down measurements of FIG. 3 to beperformed simultaneously.

In the LIDAR system 500, elements with reference designators from 510 to560 have the same function as the counterpart elements in the LIDARsystem 100 and will not be further described. The LIDAR system 500includes second CHDL 570, which is chirped in the opposite direction asthe first CHDL 510. The first and second CHDLs 510, 570 may bemultiplexed by a beam combiner 575. For example, the first and secondCHDLs 510, 570 may have orthogonal polarization, and the beam combiner575 may be a polarization beam splitter. The first and second CHDLs 510,570 may have different wavelengths, and the beam combiner 575 may be adichroic beam splitter. In any case, the beams from the first and secondCHDLs 510, 570 are combined and directed to the target 560. Thereflected beams from the target are separated by beam divider (usingpolarization or wavelength as appropriate) are directed to separatedetectors 550, 585. With this approach, the time to obtain R and dR/dtfor a pixel is T, and the 3D imaging rate is (1/T) pixels per second.Again, by minimizing the measurement time T_(M), the limitation on 3D-IRis 0.5 million pixels per second for a maximum range of 300 meters. A3D-IR of 0.5 million pixels per second, while twice the rate ofconventional coherent LIDAR systems, may still not be sufficient forsome application.

The next improvement in the conventional coherent LIDAR system resultsfrom recognition that the speed of light fundamentally imposes a delayor latency in the measurement, rather than a restriction on the imagingrate. The improvement to overcome the speed of light limit is to notreset the chirp after every pixel, but instead to use a chirp withlonger extent T′ that spans a number of (scanned) pixels as shown inFIG. 6. The example of FIG. 6 illustrates the chirp extended over threepixels, with the reflection from each pixel measured during a respectivemeasurement time. The previously described state machine CHDL canprovide a precisely controlled linear chirp over a frequency rangesufficient to allow a single chirp to span hundreds of scanned pixels.In this case, the effective 3D-IR≈1/T_(M), which is solely limited bythe measurement time and not the round-trip time to the target.Different pixels are effectively measured using chirped waves withdifferent optical frequencies, which eliminates any ambiguity in theirrange measurements, and allows faster measurements than are possiblewith the conventional coherent LIDAR systems.

As shown in FIG. 6, the returns from different pixels can have differenttime delays and Doppler shifts. This does not pose a major problem, andcan be accounted for by straightforward methods. In one approach toresolving the measurements of multiple pixels, a detector that measuresthe return signal from multiple pixels as the illumination beam isscanned across them is used in conjunction with sliding-window Fouriertransforms that resolve the ambiguity in the data processing. In anotherapproach, a multiplicity of detectors is used along with thebeam-scanning element. This embodiment takes advantage of the fact thatthe returned beam “lags” the illumination beam in a scanning system,creating a spatial separation between the two. The spatial separation islarger for farther ranges compared to nearer targets, and this fact isexploited by using a multiplicity of spatially staggered detectors whereeach one only measures the return signal from a subset of target ranges.

LIDAR systems impose a stringent requirement on the number of photonsthat need to be collected by the coherent receiver in order to achieveaccurate measurements of range and range-rate. The number of photonscollected is determined by the transmitted laser power, the reflectivityof the target, and the size of the receiver collection optics.Long-range (i.e., longer than 100 meters) LIDAR systems benefit from theuse of high output powers (e.g., 100 mW to 10 W) to minimize the sizeand complexity of the collection optics used in the coherent receiver.However, long-range coherent LIDAR systems also require very narrowlaser line width, which is generally incompatible with high laser outputpower. Semiconductor lasers with narrow line widths typically haveoutput powers less than 100 mW.

A semiconductor laser may be used in conjunction with a semiconductoroptical amplifier (SOA) or flared tapered amplifier in order to achievethe desired higher output powers while maintaining the required narrowline width. The output of a narrow line width master oscillator, whichmay be a state machine CHDL, may be fed into an optical amplifier,typically in a semiconductor medium. However, the optical and spectralproperties of the oscillator can be affected by optical feedback effectsand coupling of amplified spontaneous emission (ASE) from the amplifiersection to the oscillator, which can dramatically increase the linewidth. Thus, a feedback barrier may be disposed between the CHDL and theamplifier to ensure that the line width and other properties of the CHDLare not affected by feedback from the amplifier section. The feedbackbarrier may be, for example, an optical isolator or an attenuator. Amaster-oscillator power-amplifier (MOPA) laser with a broad-area orflared/tapered amplifier can provide single-mode operation at high(i.e., greater than 10 W) output power on a single integratedsemiconductor chip.

FIG. 7 is a schematic diagram of a narrow line width, high power, MOPAlaser 700 suitable for use in long range coherent LIDAR systems. TheMOPA laser 700 includes a feedback insensitive oscillator 710, anoptional feedback barrier 715, a preamplifier 720 and a taperedamplifier 730. The feedback insensitive oscillator, which may be statemachine CHDL as previously described, includes a laser cavity 714sandwiched between a reflector 712 and a high reflectivity output mirror716. The frequency chirp of such an oscillator can be controlledprecisely using the techniques described above. Insensitivity to opticalfeedback is achieved by increasing the reflectivity of the output mirror716, which ensures that most of the light fed back towards theoscillator 710 from the preamplifier and amplifier 720, 730 does notaffect the oscillation in the laser cavity 714. Increasing thereflectivity of the output mirror effectively incorporates the feedbackbarrier 715 into the output mirror 716. The increase in the reflectivityof the output mirror 716 will reduce the power output of the oscillator710. The reduction of oscillator output power is made up by the use ofthe preamplifier 720 between the oscillator 710 and the flared/taperedamplifier 730 to boost the optical power. The same effect of reducingthe amount of optical power fed backwards into the laser can also beachieved using an optical loss element (such as a coupler/splitter or anabsorbing section) as the feedback barrier 715. This technique alsoreduces the optical power output of the oscillator, which can becompensated by the gain of the preamplifier 720.

With few exceptions, LIDAR systems can be segregated into staringsystems and scanning systems. In a staring LIDAR, the transmitted laserbeam illuminates the entire scene to be imaged. Reflections from theentire scene are imaged onto a detector array, with each detectorelement in the array corresponding to respective pixel in the scene. Astaring coherent LIDAR must spread the reflected and LO beam over theentire detector array, which leads to insufficient signal-to-noise ratiounless the available laser power is very high. Thus, coherent LIDARsystems typically use an optical system to scan the transmitted beansequentially over the scene.

A coherent receiver for a coherent LIDAR system employing a scanningtransmitted beam has a fundamental architectural challenge. Atransmitted beam having a beam diameter D0 is scanned across the scenewithin a wide field of view Θ. The scanning is typically performed intwo dimensions. However, the subsequent figures only show scanning inone direction for ease of representation. The same design can be easilyextended to two-dimensional scanning. The size of the transmitted beam,D0, is determined by the size of the scanning optic and the requiredangular resolution (typical values of D0 are 1-3 mm). The coherentreceiver has to modally overlap the received photons from the targetwith the LO wave on a photodetector. One solution for the coherentreceiver is to use an imaging lens that images the entire field of viewΘ on a fixed detector or detector array, and illuminate the entiredetector area, whether it is a single large detector or a detectorarray, with the LO at all times. However, since only a fraction of thefield of view is imaged at any given time (this fraction can be 1/10⁵ orsmaller), this leads to an inefficient use of LO power and detectorarea, and can result in very poor signal to noise ratio due to LO shotnoise. Thus the receiver in a typical coherent LIDAR is typicallyscanned along with the transmitted beam. In other words, the LO beamneeds to be mode-matched with the return beam from the scene asdifferent parts of the field of view are illuminated.

The effective collection aperture of the coherent receiver, D1, isdictated by the requirement to collect enough photons from the target tomake a high-SNR measurement. With a sufficiently high-power laser,D1=D0, which is to say the transmitted beam diameter and the receivercollection aperture may be the same. In this case, the LIDAR opticalsystem can be a simple “cat's-eye” configuration (so-called because thetransmitted and reflected light propagate in opposing directions alongthe same optical path, as is the case with light reflected from the eyesof a cat) where the return beam from the target retraces the opticalpath of the transmitted beam, as in the coherent LIDAR system 800 shownin FIG. 8.

In the LIDAR system 800, elements with reference designators from 810 to855 have the same function as the counterpart elements in the LIDARsystem 100 and will not be further described. The laser beam (other thanthe fraction extracted for the LO) is output from the circulator 830 andexpanded/collimated by lens 870 to form an output beam having diameterD0. The lens 870, and all lenses in subsequent drawings, are depicted assingle refractive lens elements but may be any combination ofrefractive, reflective, and/or diffractive elements to perform therequired function. The output beam impinges upon scanning mirror 875,which can be rotated about an axis 880. The scanning mirror 875 may be,for example, a MEMS (micro-electro-mechanical system) mirror capable ofhigh speed scanning. Rotation of the scanning mirror causes the outputbeam to scan across the scene through a field of view θ.

Light reflected from the target impinges upon the scanning mirror 875and is directed to the optic 870. Optic 870 captures (i.e., focuses) thereflected light, which is directed to the circulator 830. The capturedreflected light exits the circulator 830 and is combined with the LObeam and detected as previously described. The diameter D0 of thetransmitted beam and the collection aperture of the receiver are definedby the optic 870. Increasing the diameter of the transmitted beamcorrespondingly increases the diameter of the receiver aperture (andthus the number of received photons) at the expense of increasing thesize of the optic 870 and the scanning mirror 875. The requirement forhigh speed scanning over the field of view limits the allowable size ofthe mirror. The transmitted beam diameter of scanning LIDAR systems istypically limited to 1 mm to 3 mm.

In the absence of a very high-power laser, the necessary diameter of thereceiver aperture D1 is preferably larger than the transmitted beamdiameter D0, and the challenge is to ensure mode overlap between thereceived beam and the LO beam at the detector with sufficientsignal-to-noise ratio.

FIG. 9 is a schematic diagram of an optical system 900 for a scanningcoherent LIDAR system in which a diameter D1 of a receiver collectionaperture may be substantially larger than a diameter D0 of a transmittedbeam 930. A first lens 910 receives light from a CHDL and forms acollimated beam with a diameter D0. The collimated beam impinges on ascanning mirror 915 which is rotated to cause the beam to scan over atotal scan angle Θ. A second lens 940 having a diameter D1, where D1>D0,receives reflected light 935 from a target scene and forms an image ofthe target scene on a detector array or a single detector. In eithercase, an area of the detector array or single detector is equal to orlarger than the scene image formed by the second lens 940. The use of adetector array instead of a single detector ensures that the detectorhas a low enough capacitance to achieve the required bandwidth. Eachdetector in a detector array corresponds to a pixel in the scene andonly one detector in the array is active for any given scan angle. Atany given instant, the reflected light 935 received by the second lens940 originates at a point in the target scene that is illuminated by thetransmitted beam 930. Thus, as the angle of the transmitted beam 930 ischanged, the received light is focused to a spot that movies laterallyin the focal plane of the second lens 940 (i.e., across the plane of thedetector 945). In systems with two-dimensional scanning of thetransmitted beam, the spot of received light will scan in two dimensionsacross the detector 945. Since the target is illuminated with a smallerbeam than the collection optic, the spot size at the plane of thedetector for a given angle of illumination is larger than theresolution-limited spot size of the lens.

To achieve coherent detection of the reflected light, it is necessarythat the LO beam and the received light be superimposed at the detector.It is possible to achieve this with an LO beam that illuminates theentire detector 945. Since the received light forms a spot that scansacross the detector 945 as the transmitted beams is scanned across thescene, it is advantageous for the LO beam to instead scan across thedetector 945 in a corresponding manner. To this end, a portion of thetransmitted beam is extracted by a tap beam splitter 920 to form the LObeam 925. The LO beam is then combined with the received light 935 by asecond beam splitter 950. Since the LO beam 925 is extracted from thetransmitted beam 930 as it is scanned, the angle of the LO beam changesin conjunction with the scanning of the transmitted beam. The tap beamsplitter 920 and beam splitter 950 effectively form a corner reflectorsuch that the LO beam is parallel to the received light when the LO andreceived beams are combined. Thus, the second lens 940 focuses the LObeam to a spot that superimposed on the spot of received light at thedetector 945.

FIG. 10 is a schematic diagram of another optical system 1000 for ascanning coherent LIDAR system in which a diameter D1 of a receivercollection aperture may be substantially larger than a diameter D0 of atransmitted beam 1030. A first lens 1010 receives light from a CHDL andforms a collimated beam with a diameter D0. The collimated beam impingeson a scanning mirror 1015 which is rotated to cause the beam to scanover a total scan angle Θ. A second lens 1040 having a diameter D1,where D1>D0, receives reflected light 1035 from a target scene and formsan image of the target scene on a detector array or a single detector aspreviously described. At any given instant, the reflected light 1035received by the second lens 1040 forms a spot that scans across thedetector 1045 as previously described in conjunction with FIG. 9.

A third lens 1060 and a fourth lens 1065 form a 1:1 telescope thatrelays the transmitted beam 1030 from the scanning mirror 1015 to thescene. A portion of the transmitted beam is extracted by a tap beamsplitter 1020 between the third and fourth lenses 1060, 1065 to form theLO beam 1025. The LO beam and the received light 1035 are combined by asecond beam splitter 1050. A relay lens 1070 in the path of the LO beamfocuses the LO beam to a spot at the plane of the detector 1045. Whenthe focal lengths of the third lens 1060 and the further lens 1065 areequal to the focal length of the second lens 1040, the focused spot ofthe LO beam will have the same size as the focused spot of receivedlight.

While FIG. 9 and FIG. 10 illustrate coherent LIDAR systems usingseparate optical paths for the transmitter and receiver, a system with asingle set of optics in the cat's-eye configuration minimizes thecomplexity of the photodetector and associated electronics complexity.However, to allow the use of a large receiver aperture, the illuminationbeam needs to be transformed from a first beam diameter D0 (which can beconstrained by the practical size of the scanning mirror) to a secondbeam diameter D1>D0 (and vice versa for the reflected light returningthrough the same optical path) without compromising the total field ofview Θ.

The number of optical modes, or unique angular positions, a beam canassume within a field of view is determined by the angular resolution ofthe beam. A beam with diameter D0 has an angular resolution ˜λ/D0(ignoring constant scaling factors of order unity throughout thisdiscussion). Therefore a beam with diameter D0 can be scanned to fillout a certain number of optical modes N0˜Θ*D0/λ within the field of viewΘ. When a scanned beam of diameter D0 is optically transformed into anew beam with a diameter D1, the angular resolution of the new beam willbe ˜λ/MD0, where M=D1/D0 is the magnification factor. However, the totalnumber of available optical modes remains constant when the diameter ofthe beam is magnified. A simple telescope that magnifies the beamdiameter from D0 to D1 will reduce the total field of view from Θ to Θ/Mto conserve the number of optical modes.

After the beam diameter is magnified to D1, the scanner will provide N0modes, with an angular resolution of λ/MD0, distributed over a field ofview of Θ/M. To recover the original field of view, the N0 modes must bespaced apart with an angular distance between adjacent modes of λ/D0,such that the N0 modes are sparsely distributed across the field of viewΘ. This means that only a portion of each scene resolution element isilluminated as the transmitted beam is scanned over the scene.Practically this means that the LIDAR system images the same total fieldof view in the same measurement time by trading off scene pixel fillfactor for an increase in the received optical power.

A mode transformer may be used to transform a set of N0 closely spacedmodes spanning a field of view Θ/M into a set of N0 modes that sparselysample a field of view Θ. For example, a mode transformer can beimplemented by coupling each available mode into a respective singlemode optical fiber, and then moving the fibers apart from each other to“sparsify” the modes. A collimating lens can then be used to convert thelight from each optical fiber into a D1-sized beam.

Another practical embodiment of a mode converter uses a microlens array.FIG. 11 is a schematic diagram of an optical system 1100 for a scanningcoherent LIDAR system in which the available modes are distributedsparsely over the entire field of view Θ. A first lens 1110 receiveslight from a circulator, such as the circulator 130 in FIG. 1, and formsa first collimated beam 1112 with a diameter D0. The first collimatedbeam 1112 is incident on a scanning mirror 1115 which rotates to scanthe first collimated beam through a scan angle corresponding to thefield of view Θ. A second lens 1120 receives the scanning beam from thescan mirror 1115 and creates a moving converging beam 1122 that forms afocused spot at the focal plane of the second lens. A microlens array(MLA) 1030 is placed at or proximate this focal plane, with a diameterof each microlens matched to the spot size formed by converging beam1122. Each element of the microlens array 1030 then converts (furtherfocuses) the spot incident on it, thereby creating a sparse array ofsmaller moving spots 1132 at an image plan 1135 of the MLA. A third lens1040 then collimates light from the array of smaller spots into ascanning second collimated beam 1142 having a diameter D1>D0. Theeffective speeds (i.e., focal length divided by diameter, or f-number)of the microlenses and the third lens are matched and are faster (lowerf-number) than an effective speed of the second lens 1120. Reflectedlight received from the scene follows the reverse path to return to thecirculator. Thus, the optical system 1100 creates an array of spots thatsamples the full angular field of view Θ at the angular resolution ofthe initial beam of size D0, while collecting more target photonscorresponding to the larger beam size D1.

When the focal lengths of the second lens 1120 and the third lens 1140are the equal, the field of view Θ will be equal to the beam scan angle.When the focal lengths of the second and third lenses 1120, 1140 areunequal the field of view can be expanded or compressed compared to thebeam scan angle.

The optical system 1100 of FIG. 11 includes a wide-field of view thirdlens 1140 to form a scanning beam of diameter D1 and to collect photonsfrom the full field of view. FIG. 12 is a schematic diagram of anoptical system 1200 in which the large lens 1140 of FIG. 11 is replacedby three smaller lenses 1240A, 1240B, 1240C that each create D1-sizedbeams that only scan over one-third of the total field of view Θ.Folding mirrors 1250A, 1250B, 1250C, 1250D are used to “combine” thefields-of-view of the three lenses 1240A, 1240B, 1240C to achieve thefull angular field of view Θ. Other elements of the optical system 1200have the same function as the corresponding elements in the opticalsystem 1100. The optical system 1200 has the advantage of using smalleroptics and reducing overall system complexity by taking advantage of thefact that the size and complexity of optical elements tend to scalenonlinearly with the field of view. A different number of smaller lenses(rather than three), or different relay optics can be used instead(other than folding mirrors), to achieve the same desired result. Inaddition, by placing the folding mirrors 1250B, 1250C before themicrolens array 1230, the large microlens array 1230 may be replaced bythree smaller microlens arrays to achieve the same desired result, usingappropriate relay optics.

Coherent LIDAR measures the amplitude and phase of the electromagneticfield reflected from the target. This reflected field is stronglyinfluenced by surface irregularities on the target within theilluminated spot. These irregularities result in random variations inthe amplitude and phase of the reflected field, commonly known asspeckle. In most practical cases, the intensity of the reflected fieldhas an exponential probability distribution and the phase has a uniformprobability distribution. This means that even a bright target canoccasionally have a low intensity and may be below the detectionthreshold of the LIDAR system. The spatial scale of the specklevariations is given by the resolution of the receiver optics. Forexample, in a conventional coherent LIDAR system, if the angularresolution of the LIDAR system is 0.1 degrees, the amplitude and phaseof the speckle pattern change every 0.1 degrees.

The probabilistic nature of target reflections also occurs in RADARsystems, and techniques for RADAR detection of so-called “fluctuatingtargets” have been developed. These techniques rely on multiple RADARmeasurements of the target to overcome the target strength (and phase)fluctuations. These measurements are then coherently or incoherently“integrated” to overcome the negative effects of target fluctuations.Integration refers to the process of combining multiple measurements toextract range and/or Doppler measurements, and can provide a higherprobability of detection of the target for a given signal to noise ratioof the measurements. The process of incoherent integration ignores theoptical phase of the reflected field, whereas coherent integrationutilizes the phase information. The process of integration works best ifthe multiple measurements being integrated are in some way uncorrelatedfrom each other, so that the target fluctuations can be “averaged” out.Speckle in a LIDAR system is comparable to a fluctuating target in aRADAR system, and similar mathematical techniques can be applied tomitigate the effects of speckle. However, the goal of a high-speedcoherent LIDAR system is to obtain range and Doppler information fromevery scene pixel in a single scan of the field of view. Thusintegration over multiple scans, as used in RADAR systems, cannot bedirectly applied to a LIDAR system.

The key to speckle mitigation in LIDAR systems is to obtain multiplemeasurements over each scene pixel during each scan of the field ofview, and then coherently or incoherently integrate (combine) thesemeasurements to mitigate target fluctuations (speckle). FIG. 13A andFIG. 13B illustrate two approaches to obtain the multiple measurements.The common idea behind both approaches is to divide the pixel into Nsubpixels, perform separate LIDAR measurements on the subpixels, andcoherently or incoherently integrate these measurements to provide acomposite measurement for the pixel.

In the first approach shown in FIG. 13A, each scene pixel 1310 ispartitioned into N subpixels 1320, where N is an integer greater thanone, that are measured sequentially. In the example of FIG. 13A, N=3.Each subpixel is measured using an illumination beam that is scannedacross the pixel during the pixel measurement time T_(M) (as defined inFIG. 1). The angular size of the illumination beam determines thesubpixel size. A one-dimensional scan is shown in FIG. 13A forsimplicity, but other scanning patterns are possible. N separate LIDARmeasurements are sequentially performed over these subpixels (eachmeasurement takes time T_(M)/N), and these measurements are coherentlyor incoherently integrated to determine the range and/or range-rate ofthe pixel. A beam with a narrower angular size than the desired pixelsize can be implemented in multiple ways, e.g., by using a scanningelement with a larger aperture in the LIDAR system, or by using amagnifying optic such as a telescope or a diffraction grating toincrease the size of the beam while using a scanner with a smallaperture. Note that the (near-field) size of the beam at the LIDARtransmitter/receiver is inversely proportional to the angular size ofthe beam in the far field (i.e., at the target).

Alternatively, the LIDAR measurements on N subpixels within a pixel canbe performed simultaneously and in parallel, as illustrated in FIG. 13B.In this example, N=4. The N subpixels are simultaneously illuminated andimaged on different photodetectors to obtain N different LIDARmeasurements (range and/or range-rate). In this case, each measurementis performed over the full pixel measurement time T_(M). The Nmeasurements are coherently or incoherently integrated to provide acomposite range and/or range-rate of the pixel.

A hybrid of the techniques shown in FIG. 13A and FIG. 13B is alsopossible. For example, a pixel may be divided into four subpixels (as inFIG. 13B) which are scanned in two horizontal (as shown in FIG. 13A)steps by two vertically-offset beams.

A third approach to speckle mitigation is to take advantage of thewavelength dependence of the amplitude and phase of the speckle pattern,Multiple independent measurements of a scene pixel may be obtained byilluminating the pixel with light at different wavelengths. LIDARmeasurements performed at different wavelengths are coherently orincoherently integrated (combined) to determine the range and/orrange-rate at that pixel, while mitigating the effects of speckle. Forexample, the beams from two lasers having different wavelengths may becombined (as previously shown in FIG. 5) and scanned over a scene. Thereflected light may be separated into two components by awavelength-selective beam splitter (as shown in FIG. 5) and detected byrespective photodiodes. A data processing function may coherently orincoherently combine the outputs from the two photodetectors todetermine the range and/or range-rate of each pixel. This technique canbe easily extended to three or more lasers with different wavelengths.

FIG. 14 is a schematic diagram of a coherent LIDAR system 1400 thatperforms simultaneous measurements of N subpixels within each pixel. Thesystem 1400 is similar to the LIDAR system 800 of FIG. 8, but with Nbeams propagating in parallel though much of the system.

The output of a CHDL 1410 is divided into N parallel beams. A tapcoupler 1420 is used to extract a small fraction of each beam as arespective LO beam. The majority (typically >90%) of each CHDL beam isdirected toward a target via an N-channel circulator 1430. The N-channelcirculator may be, for example, implemented using discrete opticalelements (e.g. known combinations of polarizing beam splitters and aFaraday rotator) such that N beams can pass through the circulator inparallel. A lens 1470 converts the N output beams 1435 from thecirculator 1430 into N collimated output beams at slightly differentangles such that the N beams illuminate N subpixels within a targetpixel, as shown in FIG. 13B. The N output beams are scanned across afield of view by a scanning mirror 1475.

The lens 1470 collects light reflected from the target (not shown) andforms the reflected light into N received light beams 1440. The receivedlight beams separated from the output beams by the circulator 1430 andare combined with respective LO beams by N couplers or beamsplitters1445. The combined LO waves and the received light beams from the targetare respectively incident on N photodetectors 1450. The N photodetectors1450 provide respective measurements indicative of the range and rangerate of respective subpixels, as previously described. A processor (notshown) coherently or incoherently combines the N subpixel measurementsto provide a composite range and/or range rate measurement for thepixel.

FIG. 15 is a block diagram of a scanning optical 1500 system for a LIDARthat that performs simultaneous measurements of N subpixels within eachpixel. The optical system 1500 is similar to the optical system 1100 ofFIG. 11, but with N beams propagating in parallel though much of thesystem.

A first lens 1510 receives N beams from a circulator, such as thecirculator 1430 in FIG. 14, and forms N collimated beams with diameterD0. The collimated beams are incident on a scanning mirror 1515 atslightly different angles (not shown). The scanning mirror 1515 rotatesto scan the N collimated beams through a scan angle corresponding to thefield of view Θ. A second lens 1520 receives the N scanning beams fromthe scan mirror 1515 and creates a moving array of spots at its focalplane. A microlens array (MLA) 1530 is placed at this focal plane. The Nbeams from the circulator are configured such that each beam illuminatesa single microlens in the MLA, and N microlenses are togetherilluminated. As a result, N smaller spots are formed at the image plane1535 of the MLA. These N spots are formed into N slightly offset beamsby the third lens 1540 that simultaneously illuminate N subpixels of thescene. A single beam from the circulator, with a different size, can beused instead of the N beams, to achieve the desired goal ofsimultaneously illuminating N microlenses after passing through thefirst lens 1510, scanning element 1515 and second lens 1520.

The reflected light from these N subpixels is received by the third lensand propagates through the scanning optical system 1500 in the reversedirection, which results in N separate beams returned to the circulator,as was the case in the LIDAR system 1400 of FIG. 14. As previouslydescribed in conjunction with FIG. 14, the N beams of reflected lightare then combined with respective LO beams. The combined beams arerespectively incident on N photodetectors that provide respectivemeasurements indicative of the range and range rate of respectivesubpixels. A processor (not shown) coherently or incoherently combinesthe N subpixel measurements to provide a composite range and/or rangerate measurement for the pixel.

A high-resolution coherent LIDAR or imaging system has two importantcomponents: i) a swept-frequency laser or “chirped laser” with a largefrequency sweep range B to provide high axial resolution at the range ofinterest; and ii) a technique to translate the one-pixel (fixed (x,y))measurement laterally in two dimensions to obtain a full 3-D image.

Coherent LIDAR systems for 3-D imaging typically rely on the scanning ofa one-pixel measurement across the scene to be image. Examples ofscanning LIDAR systems were previously shown in FIGS. 8, 9, 10, 11, 12,14 and 15.

An alternative to scanning LIDAR systems is a staring system consistingof a swept frequency laser, and a detection technique that is capable ofcapturing or measuring the 3-D image of a scene, or a multi-pixelportion of a scene concurrently. Such a system has the potential to beinexpensive, robust, and contain no moving parts.

FIG. 16A is a block diagram of one pixel of the detection side of astaring coherent LIDAR. A local oscillator beam having beam power ofP_(LO) and reflected light from a target pixel having power P_(tar) arecombined by a beam splitter. A balanced detector pair 1610 may be usedto obtain a high dynamic range. The output of the balanced detector 1610is amplified using a transimpedance amplifier (TIA) 1615, digitizedusing an analog-to-digital converter (ADC) 1620, and the spectrum of thephotocurrent signal is calculated by a Digital Signal Processor (DSP)1625 using a Fourier transform. The DSP 1625 may be on the chipcontaining the TIA 1615 and ADC 1620, or external to the chip. While thebasic output from a measurement of a single pixel are two valuescorresponding to the strength (reflectivity) and range of the target atthe pixel, further data processing to enhance the signal (e.g.,thresholding, filtering, interpolation etc.) are also possible.

This single pixel detector described above is extended to an N-elementarray (one-dimensional or two-dimensional) to implement the full-field3-D imaging system as shown in FIG. 16B and

FIG. 16B is a block diagram of a receiver array for full-field 3-Dimaging where K pixels of the scene are simultaneously imaged. Twoaligned detector arrays 1660A 1660B are used to perform the opticalbalancing. Each detector array contains K detectors to performsimultaneous measurements on K pixels of the scene. The detector arrays1660A, 1660B may be located on separate chips/wafers or on a singlewafer (using an external flip mirror or equivalent to align the twooutputs of the beam splitter on the two detector arrays). Alternatively,other balancing approaches may be implemented with a singlephotodetector array, e.g., using phase shifters, or polarization opticsand pixelated polarizers, on adjacent pixels to introduce a it-phaseshift on the LO or the target beam. The addition of a it-phase shift inone of LO or target arms simulates a balanced detection scheme. Anunbalanced single detector array can also be used to perform themeasurement, but this may limit the dynamic range of the system. Thedynamic range can be improved by subtracting the common mode currentfrom the photodetector using an external current source.

The output of the detector arrays (whether K single detectors or 2Kdetectors to form K balanced pairs) is amplified with an array of TIAs1665, digitized with an array of ADCs 1670, and one or more signalprocessors 1675 performs the Fourier transforms (typically using the FFTalgorithm), the detection algorithms, and the input/outputcommunication. The output of receiver array is typically two “images”(i.e., two values per pixel) corresponding to the depth map and theintensity of the reflections. Measuring the scene with alternate up- anddown-chirps will also allow measuring the range rate of each scenepixel. Further data processing algorithms may also be implemented by thesmart detector array or external processors.

The various electronic components (detectors, TIAs, ADCs and signalprocessors) may be fully integrated on the same wafer substrate, e.g.,silicon electronics with silicon or silicon-germanium detectors, orIII-V semiconductor detectors and electronics. Alternatively, hybridintegration may be used where dies with different functionalities(detectors, amplifiers, mixed-mode circuits etc.) are incorporated onthe substrate using pick-and-place techniques. Different functionalelements may be spatially separated as shown schematically in thisfigure, or may be implemented as “composite pixels” that incorporatethese different functional elements in close proximity to each other asshown in FIG. 17.

FIG. 17 is a block diagram of an alternative implementation of areceiver array 1700 where the detectors 1710, TIA 1715, ADC 1720, andother electronic components are integrated, monolithically or usinghybrid integration, in close proximity to form a series of functionalunit cells 1705. As shown in FIG. 17, two adjacent detectors 1710 areused for balancing. This approach can be readily modified to use asingle detector per unit cell, or the unit cell may only contain somefunctional blocks, with the remaining functions being performedseparately.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A light detection and ranging (LIDAR) system,comprising: a laser source to emit one or more optical beams; a scanningoptical system to scan the one or more optical beams over a scene andcapture reflections of the one or more optical beams from the scene; anda measurement system to measure the reflections of the one or moreoptical beams from the scene, wherein the measurement system divides thescene into a plurality of pixels, and for each pixel of the plurality ofpixels, the measurement system makes independent measurements of thereflections from N subpixels within the pixel, where N is an integergreater than 1, and combines the measurements of the reflections fromthe N subpixels to determine range and/or range rate for the pixel. 2.The LIDAR system of claim 1, wherein for each pixel of the plurality ofpixels, the measurement system combines measurements of the reflectionsfrom the N subpixels independent of phases of the reflections from themultiple subpixels.
 3. The LIDAR system of claim 1, wherein for eachpixel of the plurality of pixels, the measurement system measures thereflections from at least some of the N subpixels sequentially.
 4. TheLIDAR system of claim 1, wherein for each pixel of the plurality ofpixels, the measurement system measures the reflections from at leastsome of the N subpixels concurrently.
 5. The LIDAR system of claim 1,wherein the laser source is a frequency modulated laser, wherein anoptical frequency of the one or more optical beams varies in accordancewith a periodic frequency versus time function.
 6. The LIDAR system ofclaim 5, wherein the periodic frequency versus time function is one of asawtooth function and a triangle function.
 7. The LIDAR system of claim5, wherein the measurement system measures the reflections from multiplesub-pixels during each period of the periodic frequency versus timefunction.
 8. The LIDAR system of claim 5, wherein: the scanning opticalsystem causes N scanned optical beams to respectively illuminate the Nsubpixels within each pixel, and the measurement system comprises, foreach of the N scanned optical beams: an optical combiner that combines arespective local oscillator beam with the reflection from the respectivesubpixel, and a photodetector that receives the combined localoscillator beam and reflection from the optical combiner and convertsthe combined local oscillator beam and reflection into an electricalsignal.
 9. The LIDAR system of claim 8, wherein the measurement systemfurther comprises: a data processor to determine the range and/or rangerate for each pixel based on the electrical signals from the respectivephotodetectors for the N scanned optical beams.
 10. The LIDAR system ofclaim 8, wherein the measurement system further comprises: a multibeamoptical circulator configured to: receive the reflections from the Nsubpixels from the scanning optical system and direct the reflections tothe respective optical combiners.
 11. The LIDAR system of claim 10,wherein the one or more optical beams emitted by the laser source are Noptical beams, the measurement system includes N optical taps, eachoptical tap to extract a portion of a respective one of the N opticalbeams to provide a respective local oscillator beam, and the multibeamoptical circulator receives the N optical beams from the respectiveoptical taps and directs the N optical beams to the scanning opticalsystem.
 12. The LIDAR system of claim 11, wherein the scanning opticalsystem comprises: a lens to receive the N optical beams from themultibeam optical circulator and collimate the N optical beams to form Ncollimated beams; and a scanning mirror that receives the N collimatedbeams from the first lens and scans the N collimated beams over thescene.
 13. The LIDAR system of claim 11, wherein the scanning opticalsystem comprises: a first lens to receive the N optical beams from themultibeam optical circulator and collimate the N optical beams to form Nfirst collimated beams; a scanning mirror that receives the N firstcollimated beams from the first lens and scans the N collimated beamsover a scan angle; a second lens to receive the N first collimated beamsfrom the scanning mirror and focus the N first collimated beams to formN respective converging beams; a microlens array disposed proximate afocal plane of the second lens, the microlens array configured such thateach of the N converging beams fills a respective microlens element ofthe microlens array, the micro lens array further configured to furtherfocus the N converging beams to form N respective focused beams; and athird lens to collimate the N focused beams to provide N secondcollimated beams directed towards the scene.
 14. The LIDAR system ofclaim 10, wherein the one or more optical beams emitted from the lasersource is a single source beam, the measurement system includes anoptical tap to extract a portion of the source beam to provide N localoscillator beams, and the multibeam optical circulator receives thesource beam from the optical tap and directs the source beam to thescanning optical system.
 15. The LIDAR system of claim 14, wherein thescanning optical system comprises: a first lens to receive the sourcebeam from the multibeam optical circulator and collimate the source beamto form a first collimated beam; a scanning mirror that receives thefirst collimated beam from the first lens and scans the first collimatedbeam over a scan angle; a second lens to receive the first collimatedbeam from the scanning mirror and focus the first collimated beams toform a first converging beam; a microlens array disposed proximate afocal plane of the second lens, the microlens array configured such thatthe first converging beam fills N microlens elements of the microlensarray, the micro lens array further configured to further focus thefirst converging beams to form N respective focused beams; and a thirdlens to collimate the N focused beams to provide N second collimatedbeams directed towards the scene.
 16. A light detection and ranging(LIDAR) system, comprising: a laser source to emit an optical beam; ascanning optical system to scan the optical beam over a scene andcapture reflections of the optical beam from the scene, wherein thescanning optical system further comprises: a first lens to collimate theoptical beam to form a first collimated beam; a scanning mirror thatreceives the first collimated beam from the first lens and scans thefirst collimated beam over a scan angle; a second lens to receive thefirst collimated beam from the scanning mirror and focus the firstcollimated beam to form a converging beam; a microlens array disposedproximate a focal plane of the second lens, the microlens arrayconfigured such that the converging beam fills a respective microlenselement of the microlens array, the micro lens array further configuredto further focus the converging beams to form a focused beam; and athird lens to collimate the focused beam to provide a second collimatedbeam directed towards the scene; and a measurement subsystem to measurereflections captured by the scanning optical system to determine rangeand/or range rate for each pixel of the scene.
 17. The LIDAR system ofclaim 16, wherein the measurement system comprises: an optical tap thatseparates a portion of the optical beam to provide a local oscillatorbeam; and an optical combiner that combines the local oscillator beamwith a reflection from the respective subpixel, and a photodetector thatreceives the combined local oscillator beam and reflection from theoptical combiner and converts the combined local oscillator beam andreflection into an electrical signal.
 18. The LIDAR system of claim 17,wherein the measurement system further comprises: an optical circulatorconfigured to: receive the optical beam from the optical tap and directthe optical beam to the scanning optical system, and receive thereflections from the from the scanning optical system and direct thereflections to the optical combiner.
 19. A method of measuring a scene,comprising: illuminating a pixel within the scene with one or moreoptical beams; independently measuring reflections of the one or moreoptical beams from a plurality of subpixels within the pixel; andcombining the measurements of the reflections from the subpixels todetermine range and/or range rate for the pixel.
 20. The method ofmeasuring a scene of claim 19, further comprising: scanning the one ormore optical beams over the scene to determine range and/or range ratefor a plurality of pixels.
 21. The method of measuring a scene of claim19, wherein the measurements of the reflections from the subpixels arecombined independent of phases of the reflections from the multiplesubpixels.
 22. The method of measuring a scene of claim 19, wherein thereflections from at least some of the N subpixels are measuredsequentially.
 23. The method of measuring a scene of claim 19, whereinthe reflections from at least some of the N subpixels are measuredconcurrently.
 24. A light detection and ranging (LIDAR) system,comprising: a laser source to emit two or more optical beams atdifferent wavelengths; a scanning optical system to scan the two or moreoptical beams over a scene and capture reflections of the two or moreoptical beams from the scene; and a measurement system to measure thereflections of the two or more optical beams from the scene, wherein themeasurement system divides the scene into a plurality of pixels, and foreach pixel of the plurality of pixels, the measurement system makesindependent measurements of the reflections from the pixel at eachdifferent wavelength, and combines the measurements of the reflectionsfrom the pixel at the different wavelengths to determine range and/orrange rate for the pixel.